The present invention relates to electronically controlled matrix printing in which text and/or graphics images are printed by a printing device on a substrate, typically a paper or plastic sheet, under control of a digital signal. The invention particularly relates to printing of this type in which the digital signal represents pixels, or image elements, which are divided into a plurality of parallel strips, with each strip being composed of a plurality of parallel rows.
Printing of this type is performed in a variety of printers, including dot matrix printers having an array of pins which can individually contact the substrate to form a dot thereon, and ink jet printers having an array of nozzles which jet individual drops of solvent-based or hot melt, i.e. thermoplastic, inks onto the substrate. Printing of this type is also performed by distributed matrix printers.
Theoretically, such printing could also be performed with an array of light emitting elements, such as LEDs, which illuminate points on a sheet of photosensitive or thermosensitive paper. However, known printers of this type conventionally have a single long row of light emitting elements which print one row of image dots at a time.
In all existing printers in which printing is effected on a succession of parallel strips on the substrate, with each strip containing a plurality of parallel rows of image elements, a stepwise relative movement, commonly referred to as an indexing movement, must be performed between the substrate and the printing device before printing each successive strip. The transporting devices for effectuating such movement are designed to give each movement step a magnitude which will assure that all of the rows of a complete image will have a constant spacing.
However, given the small magnitude of the inter-row spacing, which is typically 0.083 mm, it has not been possible to construct a transporting device which effects the movement in a sufficiently accurate and repeatable manner to avoid every type of image imperfection due to row spacing variations at the boundaries between strips.
FIG. 2 shows one example of a type of image imperfection which appears if the movement effected by the transporting device is only a few hundredths of a millimeter longer than its nominal, or desired value. While in FIG. 2, and FIG. 3, ink dots are shown surrounded by white spaces, in practice, the dots will overlap and substantially fill the inter-dot spaces. In FIG. 2, portions 2 and 4 of two strips of printed image elements, e.g. ink dots, are separated by a boundary 6. Three rows of a portion 8 of a third strip are separated from portion 4 by a second boundary 6. On the substrate, each strip covers a width L.sub.0 and the inter-row spacing, i.e. the spacing between adjacent rows of dots, has a value, l. The first strip contains dot rows 1-9 and the next strip contains dot rows 10-18. If the indexing movement performed after printing rows 1-9 has a value exactly equal to L.sub.0, image imperfections of the type described above will not occur.
However, in practice, the indexing movements will rarely have exactly the desired value. Rather, because of inherent imperfections in the transporting mechanism, and/or position detection errors, each indexing movement will be longer or shorter than the desired, or nominal, value. The magnitude of the error will, naturally, not be predictable, but a maximum value for the error magnitude can be determined. In FIG. 2, the indexing movement error between printing of the strip containing portion 2 and the strip containing portion 4 is +d, which can be assumed to be the maximum error magnitude that will occur. It can further be assumed that in a typical printer, d&lt;l.
If, as shown, the image elements represent a pattern which extends across a boundary 6, then the positioning error will result in the appearance of a visible seam, or continuous white or light line, along boundary 6.
This defect is most apparent when printing graphics images since normal text printing is performed so that the seam at each boundary between strips of image elements is located in a blank area between lines of printed text.
In order to eliminate defects of this type, it has been proposed to intentionally reduce the magnitude of the indexing movement and to use the data intended for printing the last row of one strip to control printing of both that row and the first row of the next strip. Such a technique is disclosed in U.S. Pat. No. 4,272,271 which issued to Furukawa on Jun. 9, 1981, and is shown in FIG. 3 herein, which will be described below.
While this technique will eliminate visible seams, it has the drawback of reducing the effective width of each strip, thereby resulting in an increase in the number of strips required to fill a substrate sheet and an increase in the time required to print each sheet.